net: hns3: limit some variable scope in critical data path

This patch limits some variables' scope as much as possible in
hns3_fill_desc.

Also, only set l3_type and l4_type when necessary.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Huazhong tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>